A coumarin-functionalized polyimide film for optical encryption, its preparation method, and a method for multi-level grayscale optical encoding and decoding.

Abstract

This invention relates to organic photosensitive functional materials and optical information security technology, and discloses a coumarin-functionalized polyimide film for optical encryption, its preparation method, and a method for multi-level grayscale optical encoding and decoding. The film is prepared by polycondensation of a polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin with a copolyamine and dianhydride, followed by thermal imidization. The coumarin photosensitive unit is attached to the polyimide backbone as a side group. During encoding, a grating mask covers the film, and different exposure doses are applied to different areas of the film using ultraviolet light. In the exposed areas, the coumarin side groups undergo intermolecular [2+2] photocycloid crosslinking, causing an increase in local network density and volume shrinkage, forming a multi-level depth surface relief grating. The encoded information is stored in a spatial distribution form with nanoscale grating depth. Under natural light, the information is hidden; only under directional light irradiation, the grating at different depths produces differentiated diffraction structural colors, allowing the encoded information pattern to be revealed and read.

Description

Technical Field

[0001] This invention relates to organic photosensitive functional materials and optical information security technology, specifically to a coumarin-functionalized polyimide film for optical encryption, its preparation method, and a method for multi-level grayscale optical encoding and decoding. Background Technology

[0002] Optical encryption and anti-counterfeiting technologies have significant application value in fields such as information security, product traceability, and identity authentication. Based on the different information carriers and response mechanisms, existing technologies can be mainly divided into the following categories.

[0003] Firstly, there are physical anti-counterfeiting schemes based on surface micro / nano structures, such as holographic gratings and diffraction-based optical images. These structures typically rely on precision fabrication processes like laser interference, electron beam etching, or nanoimprinting, which are costly and, once the pattern is finalized, difficult to dynamically expand the information capacity on a single device. More importantly, their optical response depends primarily on a predetermined period and morphology, making it difficult to achieve continuously adjustable multi-level grayscale information encoding in different regions of the same thin film through post-processing. Furthermore, the optical response of these pre-defined micro / nano structures is often a fixed pattern displayed at a specific angle; the information itself lacks a hidden design in spatial distribution, and its diffraction patterns can be directly photographed or scanned and copied, limiting the level of anti-counterfeiting.

[0004] Secondly, there are chemical anti-counterfeiting materials based on photochromism or fluorescence emission. These materials undergo reversible or irreversible molecular structural changes under specific wavelength excitation, resulting in color or luminescence changes. However, publicly available literature generally reports that these organic chromophores suffer from photobleaching and photofatigue problems, and their long-term stability is insufficient; moreover, their response modes are mostly threshold-type or binary switching-type, that is, there are only two states before and after illumination: "color change / no color change" or "luminescence / no luminescence". The response to the excitation dose lacks a linear and quantifiable structural change correspondence, making it difficult to construct high-density grayscale levels.

[0005] Thirdly, there is photolithography patterning technology based on photosensitive polyimide. Polyimide, due to its excellent thermal stability and mechanical strength, is an ideal functional matrix material. Currently available commercially available photosensitive polyimides mostly use diazonaphthoquinone or acrylate-based photosensitive units. Their imaging mechanism relies on the difference in solubility before and after exposure. After exposure, a wet development process is necessary to form a pattern, ultimately resulting in a "retain / remove" binary structure. This mechanism makes it difficult to directly convert continuous changes in exposure dose into a gradient structure of the film surface morphology. Therefore, there are technical obstacles to writing multi-level grayscale information within a single-layer film.

[0006] Fourth, recent literature reports the use of photosensitive polyimide or polyurethane systems containing 1,4-dihydropyridine (DHP) units. Ultraviolet light irradiation induces photo-induced dehydration of DHP and subsequent hydrogen bond rearrangement, leading to stress instability between the exposed and unexposed areas of the thin film, thereby forming a wrinkled embossed grating on the surface. Its application in holographic anti-counterfeiting has been explored. However, according to the mechanisms disclosed in these publications, grating formation mainly relies on the reassembly of non-covalent bonds (hydrogen bonds) and the resulting physical instability and wrinkling, rather than chemical cross-linking between molecular chains. Furthermore, the published research primarily focuses on using masks to form photolithographic patterns or holographic stripes; there are no reports or insights on how this system can precisely control the surface grating depth and achieve multi-level grayscale encoding by applying continuously varying exposure doses to different areas within the same thin film.

[0007] Furthermore, coumarin compounds, under long-wavelength ultraviolet light (such as 365 nm), can undergo intermolecular [2+2] photocycloaddition reactions through their C3=C4 double bonds to generate cyclobutane crosslinked structures. The degree of crosslinking increases with increasing exposure dose, exhibiting dose-dependent photoresponse characteristics, which is already known in the field. Meanwhile, the introduction of coumarin motifs as side groups into polyimides has also been observed in some studies, but its main application focuses on using linearly polarized ultraviolet light to induce molecular anisotropic orientation as an alignment layer in liquid crystal displays. The goal is to achieve directional alignment of liquid crystal molecules, rather than constructing a dose-dependent relief depth gradient on the film surface for optical encryption.

[0008] In summary, existing optical encryption materials and technologies still have technological gaps in terms of information encoding dimensions (multi-level grayscale), structural stability (high-temperature resistance), and information reading methods (hidden-dynamic display). In particular, a method for directly inducing dose-dependent relief depth changes on the thin film surface using photodimerization and applying this mechanism to multi-level grayscale optical encoding has not been reported. Therefore, this invention provides a coumarin-functionalized polyimide thin film with a specific structure and its multi-level grayscale optical encoding method to solve one or more of the aforementioned technical problems. Summary of the Invention

[0009] In view of this, the present invention provides an optical encryption film based on coumarin-functionalized polyimide, and a method for constructing a multi-level depth surface relief grating by spatially modulating exposure dose to achieve multi-level grayscale optical encoding and complete development-free process.

[0010] To achieve the above objectives, in a first aspect, the present invention provides a coumarin-functionalized polyimide film for optical encryption, which is prepared by polycondensation of a dianhydride monomer, a copolydiamine monomer and a polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin and followed by thermal imidization. The molar ratio of the dianhydride monomer, the copolydiamine monomer, and the polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin is 10:(6~8):(2~4), and the polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin accounts for 20~40% of the total diamine monomers. The polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin has the structure shown in formula (I): Formula (I) In formula (I), Ar represents a phenyl unit, and the Ar units are connected by carbon-carbon single bonds to form a polyphenyl aromatic skeleton; the curved bonds represent the linking sites, each independently selected from the ortho, meta, or para positions of the corresponding Ar unit; m and n are each independently selected from 1, 2, or 3, and l is selected from 1 or 2; the two –NH2 groups are each independently located at the ortho, meta, or para positions of the corresponding terminal Ar unit; –O–(CH2) p –Z–(CH2)q–O– is a flexible bridging chain, where p and q are each independently selected from integers 1 to 6, and Z is selected from a single bond, –O–, –S–, or –NH–. Here, “single bond” means that no atom is inserted at the Z position, i.e., (CH2). p The flexible bridging chain is directly connected to (CH2)q via carbon-carbon single bonds, and the overall structure is represented as "–O–(CH2)p". + The structure of a pure methylene ether chain is q–O–; while –O–, –S–, and –NH– represent the insertion of an ether oxygen, thioether, or secondary amine group as a heteroatom spacer at that position, respectively.

[0011] As a further preferred embodiment of the present invention, the dianhydride monomer is selected from one or more of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropyl)diphthalic anhydride, benzophenone tetracarboxylic dianhydride, and 4,4'-oxobisphthalic anhydride; the copolydiamine monomer is selected from one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenylmethane, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane.

[0012] As a further preferred technical solution of the present invention, at least one of the compounds shown in formulas (II), (III), and (IV) is provided: Equation (II) Formula (III) Formula (Ⅳ).

[0013] Formula (II) is a preferred specific compound in Formula (I) when the polyphenylene is terphenyl, the amino groups at both ends are in the para position, and the flexible ether bridge is –O–(CH2)4–O– (i.e. Z is a covalent single bond, p+q=4).

[0014] According to a second aspect of the present invention, the present invention also provides a method for preparing a coumarin-functionalized polyimide film for optical encryption as described in the first aspect, comprising the following steps: (1) Under an inert atmosphere and light-protected conditions, polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin and copolydiamine monomer are dissolved in an aprotic polar solvent, and dianhydride monomer is added in batches. Polycondensation is carried out at 0~10℃ to obtain a polyamic acid solution. (2) Coating a polyamic acid solution onto a substrate and heating to remove the solvent to obtain a precursor film; (3) The precursor film was thermally imidized by gradient heating under a protective atmosphere to obtain a coumarin-functionalized polyimide film.

[0015] As a further preferred embodiment of the present invention, the aprotic polar solvent is one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide; and / or, the maximum temperature of the thermal imidization is 250~300°C.

[0016] According to a third aspect of the present invention, the present invention also provides a multi-level grayscale optical encoding method, which utilizes the coumarin-functionalized polyimide film for optical encryption of the first aspect, comprising the following steps: covering the film with a grating mask, applying different exposure times to different regions of the film with ultraviolet light, causing intermolecular [2+2] optical ring crosslinking of the coumarin side groups in the exposed region, resulting in an increase in local network density and volume shrinkage in the exposed region, and forming a surface relief grating with different depths corresponding to the exposure dose on the film surface.

[0017] As a further preferred embodiment of the present invention, the period of the grating mask is 5~20μm; and / or, the wavelength of the ultraviolet light is 365nm, and the single exposure time is 0.5~16s.

[0018] As a further preferred technical solution of the present invention, the surface relief gratings of different depths are formed by applying different exposure doses to different areas of the thin film. The exposure doses are achieved by one or a combination of step exposure, progressive exposure, local repeated exposure, and grayscale exposure.

[0019] According to a fourth aspect of the present invention, the present invention also provides a method for reading optically encrypted information. For a thin film encoded by the multi-level grayscale optical encoding method of the third aspect, the encoded information pattern of the thin film is hidden and unreadable under natural light; when the thin film is irradiated by directional light, the differentiated diffraction structural colors generated by gratings of different depths reveal the encoded information pattern and can be read by a barcode scanning device.

[0020] According to a fifth aspect of the present invention, the present invention also provides an optical encryption or anti-counterfeiting article comprising a coumarin-functionalized polyimide film for optical encryption according to the first aspect, or comprising a film encoded using the multi-level grayscale optical encoding method of the third aspect; the article is used for information security, product traceability, identity authentication, or high-end packaging anti-counterfeiting.

[0021] Compared with the prior art, the present invention can achieve the following beneficial effects: Compared with the prior art, the present invention can achieve the following beneficial effects: (1) Dose-depth modulation type multi-level grayscale encoding: By utilizing the dose dependence of coumarin [2+2] crosslinking, the exposure dose can be spatially modulated to form surface relief gratings of different depths in different areas of the same film, thereby realizing multi-level grayscale optical encoding; unlike traditional photosensitive polyimide that can only respond in binary form, this invention can construct multi-level information on a single film, with higher information capacity.

[0022] (2) High-security anti-counterfeiting with dynamic reading of “hidden-revealed”: The encoded information exists in the form of spatial distribution of nanometer-level grating depth. Under natural light, the structural color difference of each area is not easy to distinguish and the information is hidden. It is only revealed by diffraction under directional light. The information carrier is the spatial distribution of grating depth rather than surface pattern or chemical color. Conventional photography, scanning and copying are difficult to obtain its diffraction characteristics, and the anti-counterfeiting level is high.

[0023] (3) Larger dose-depth response window based on covalent cross-linking mechanism: The surface relief grating of the present invention originates from the intermolecular covalent cross-linking formed by the photo-addition of coumarin [2+2], thereby causing structural rearrangement of the molecular chain network in the exposure area and changes in local modulus and packing density; thanks to the synergistic effect of the covalent cross-linking mechanism and the rigid polyimide matrix, the present invention can exhibit a wider dose-depth response window under the same mask exposure method, and the resulting grating relief depth can be continuously adjusted between about 13 nm and about 132 nm with the exposure dose (see Example 1 and Figure 4 The maximum relief depth of the grating obtained by this invention can reach approximately 132 nm, and the cyclobutane covalent bond is more stable than the hydrogen bond, which is beneficial for the long-term preservation and thermal stability of the encoded information.

[0024] (4) Process compatibility and environmental resistance of the polyimide matrix: The present invention adopts polyimide as the film matrix. Polyimide per se has well-known high thermal stability and chemical stability in the art, can withstand the high-temperature thermal imidization process at 250~300°C required in the preparation method of the present invention, and is suitable for long-term anti-counterfeiting applications in relatively harsh environments. Meanwhile, the present invention adopts the process sequence of "performing thermal imidization to form a film first, then performing exposure to form a grating". At the thermal imidization stage, the coumarin side groups are not yet cross-linked, which avoids thermal relaxation or disappearance of the already formed grating caused by high temperature. Combined with the flexible ether bridge structure, necessary segment mobility is reserved for the coumarin side groups in the rigid polyimide main chain, so that the [2+2] photocrosslinking in the solid film can be effectively carried out. Brief Description of the Drawings

[0025] The present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

[0026] Figure 1 is a hydrogen nuclear magnetic resonance spectrum of the 4-(trifluoromethyl)coumarin-containing polybiphenyldiamine monomer used in Example 1.

[0027] Figure 2 is a schematic diagram of the synthesis route of the polyimide for optical encryption of the present invention, which shows that pyromellitic dianhydride, 4,4'-diaminodiphenyl ether and the 4-(trifluoromethyl)coumarin-containing polybiphenyldiamine monomer are subjected to low-temperature polycondensation in N-methylpyrrolidone at a molar ratio of 10:7:3 and then undergo thermal imidization to obtain the coumarin side group-functionalized random copolyimide; in the figure, x and y represent the mole fractions of the ODA unit and the coumarin diamine unit, x+y=1, y is 0.20~0.40, and n is the degree of polymerization; the actual structure is a random copolymer structure; in Example 1, x=0.7 and y=0.3.

[0028] Figure 3 is a Fourier transform infrared spectrum of the polyimide film obtained in Example 1, which is used to confirm the imidization and the retention of the coumarin photosensitive unit.

[0029] Figure 4 is a graph showing the relationship between exposure time and grating relief depth in Example 1, which indicates that the grating depth first increases and then decreases with the extension of exposure time, and reaches the peak at about 7s.

[0030] Figure 5 shows the reading effect and AFM characterization of the multi-level grayscale coded film with the shape of the Chinese character "zhong" in Example 2: (a) an optical photograph of the information hidden state under natural light, (b) an optical photograph of the information revealed state under directional light irradiation, (c) an atomic force microscope three-dimensional topography of the junction between the 3s exposure area and the 5s exposure area, (d) an atomic force microscope three-dimensional topography of the junction between the 5s exposure area and the 7s exposure area.

[0031] Figure 6The UV-Vis absorption spectra of acetonitrile solution of 4-methylcoumarin diamine (sample B) after irradiation with 365 nm UV light for different times (0, 1, 2, 4, 8, 16, 32 min).

[0032] Figure 7 The UV-Vis absorption spectra of 4-(trifluoromethyl)coumarin diamine (sample A) acetonitrile solution after irradiation with 365 nm UV light for different times (0, 1, 2, 4, 8, 16, 32 min).

[0033] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0034] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0035] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0036] The specific preparation method of the polyphenylenediamine monomer containing 4-(trifluoromethyl)coumarin with the structure of formula (II) of this invention is as follows: Step 1. Composition of the hub module (intermediate E): In a 250 mL double-necked flask, 3,5-dibromophenol (1.00 g, 3.97 mmol), 4-aminophenylborate (1.38 g, 7.94 mmol), tetra(triphenylphosphine)palladium (0.23 g, 0.20 mmol, 5 mol%), and potassium carbonate (3.28 g, 23.8 mmol) were added sequentially, followed by tetrahydrofuran (40 mL) and water (10 mL). The mixture was stirred under nitrogen protection in the dark. The temperature was raised to 80 °C, and the reaction was allowed to proceed for 12 hours. After the reaction was complete as monitored by TLC, the mixture was cooled to room temperature. The reaction mixture was filtered, and the filter cake was washed 3–4 times with tetrahydrofuran. The filtrates were combined and extracted with saturated brine (50 mL × 3) and ethyl acetate (50 mL × 3). The organic phase was dried over anhydrous sodium sulfate, concentrated under reduced pressure, and purified by silica gel column chromatography (petroleum ether / ethyl acetate = 3:1, v / v) to give 1.01 g of a light yellow solid product (intermediate E), with a yield of 92%.

[0037] The specific reaction equation for this step is as follows:

[0038] Step 2. Preparation of the coumarin-butyl ether-bromine precursor: In a 250 mL double-necked flask, 7-hydroxy-4-(trifluoromethyl)coumarin (1.00 g, 4.35 mmol), 1,4-dibromobutane (1.86 g, 8.61 mmol), cesium carbonate (2.24 g, 6.87 mmol), cesium iodide (0.15 g, 0.57 mmol), and tetrabutylammonium iodide (0.21 g, 0.57 mmol) were added, followed by 18 mL of anhydrous tetrahydrofuran under nitrogen protection. The mixture was heated to 80 °C and reacted for 12 hours. After the reaction was complete as monitored by TLC, it was cooled to room temperature. The reaction mixture was filtered, and the filtrate was concentrated by rotary evaporation under reduced pressure at 35 °C to obtain the crude product. The crude product was washed three times with anhydrous diethyl ether and dried under vacuum at 60 °C for 12 hours to obtain 1.52 g of a pale yellow solid product (coumarin-butyl ether-bromine precursor), with a yield of 96%.

[0039] The specific reaction equation for this step is as follows:

[0040] Step 3, Module Assembly (Target Unit): In a 100 mL double-necked flask, intermediate E (0.50 g, 1.81 mmol) prepared in step 1, the coumarin-butyl ether-bromine precursor (0.66 g, 1.81 mmol) prepared in step 2, and sodium carbonate (0.57 g, 5.43 mmol) were added, followed by the addition of anhydrous acetone (20 mL) under nitrogen protection. The mixture was heated to 80 °C and reacted for 12 hours. After the reaction was complete as monitored by TLC, it was cooled to room temperature. The reaction mixture was filtered, and the filter cake was washed with a small amount of ethyl acetate. The filtrates were combined and concentrated by rotary evaporation under reduced pressure at 30 °C to obtain the crude product. The crude product was purified by silica gel column chromatography (dichloromethane / petroleum ether = 5:1, v / v), and dried under vacuum at 60 °C for 12 hours to obtain 0.92 g of the target photosensitive diamine monomer as a yellow solid, with a yield of 91%.

[0041] The specific reaction equation for this step is as follows: .

[0042] The proton NMR spectra of the monomers obtained above are shown in the figure. Figure 1 The chemical shifts, integrals, and splitting of the characteristic protons in the spectrum are consistent with the target monomer structure, confirming that the obtained monomer structure is correct.

[0043] Based on the polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin with the structure of formula (II) prepared above, the present invention will be further described in detail below.

[0044] Example 1: Preparation of coumarin-functionalized polyimide film, the synthesis equation is as follows: Figure 2As shown.

[0045] Step (1), Preparation of polyamic acid solution The molar ratio of PMDA:ODA:coumarin-functionalized diamine monomer with formula (II) is 10:7:3. The target polyamic acid solid content is 15wt%, and the polymerization solvent is anhydrous N-methylpyrrolidone (NMP). The specific dosage of each component is as follows: ODA (M=200.24g / mol) 0.1402g (0.700mmol), coumarin-functionalized diamine monomer (M=560.57g / mol) 0.1679g (0.300mmol), PMDA (M=218.12g / mol) 0.2181g (1.000mmol); the total amount of anhydrous NMP is 2.98g, of which 2.40g is used for diamine dissolution and 0.58g is used for PMDA rinsing.

[0046] Under light-protected conditions, 0.1402 g of ODA and 0.1679 g of coumarin-functionalized diamine monomer of formula (II) were accurately weighed into a clean, dry 25 mL single-necked flask. The weighing container was rinsed in portions with 2.40 g of anhydrous NMP (dried by molecular sieve drying and dehydrated by vacuum distillation) and the entire amount was transferred to the flask. The flask was placed in a 5°C ice-water bath and magnetically stirred until the solid was completely dissolved, yielding a homogeneous and transparent diamine / NMP solution. 0.2181 g of PMDA was added to the low-temperature diamine solution in three equal portions, every 5 minutes, for a total time of 15 minutes, maintaining a constant temperature of 5°C and stirring at a uniform speed throughout. The remaining 0.58 g of anhydrous NMP was then rinsed into the PMDA weighing container and added to the flask. The flask was then sealed with a ground glass stopper and polymerized under light-protected conditions in a 5°C ice-water bath for 12 hours with continuous stirring, yielding a pale yellow, homogeneous, viscous coumarin-modified polyamic acid stock solution. Transfer the stock solution to a brown, light-proof bottle, degas under vacuum at room temperature for 2-4 hours, seal, and freeze at -18°C for later use.

[0047] Step (2), Preparation of precursor membrane The above-mentioned polyamic acid solution was spin-coated onto a clean glass substrate that had been pretreated with ultraviolet light for 5 min. The spin-coating process was divided into two stages: pre-spinning at 500 r / min for 10 s and main spinning at 2500 r / min for 30 s. After spin-coating, the solvent was evaporated by heating at 70 °C for 1 h to obtain a polyamic acid precursor film with a film thickness of about 3~5 μm.

[0048] Step (3), thermal imidization The precursor membrane was placed under a nitrogen atmosphere and heated sequentially at a rate of 5 °C / min, followed by the following gradient and holding temperature: heated to 50 °C and held for 200 s, heated to 100 °C and held for 1000 s, heated to 150 °C and held for 1000 s, heated to 200 °C and held for 1000 s, heated to 250 °C and held for 1500 s, and heated to 300 °C and held for 1500 s. After cooling to room temperature, the membrane was removed, allowing the polyamic acid to undergo intramolecular cyclization and dehydration during the heating process to form an imide ring, ultimately yielding a coumarin-functionalized polyimide film.

[0049] The obtained polyimide film was characterized structurally and its properties were tested, as follows: 1) Structural verification (FT-IR): The Fourier transform infrared spectrum of the polyimide film obtained in this embodiment is as follows: Figure 3 As shown. The spectrum is at 1772 cm⁻¹. - ¹and 1724cm - The asymmetric and symmetric stretching vibration absorption of the imide carbonyl group appears at ¹ (characteristic double peak of imide I), at 1374 cm⁻¹. - The C–N stretching vibration absorption of the imide ring (imide II) appears at position ¹, at 716 cm⁻¹. - The carbonyl bending vibration absorption (imide IV) at position ¹ indicates that the polyamic acid has been fully cyclized and imidized; among which, 1724 cm⁻¹… - The absorption at ¹ also involves the stretching vibration of the coumarin lactone carbonyl group, 1608 cm⁻¹ - The ¹ region represents the absorption of the C=C conjugated stretching vibration of the aromatic ring, including the C3=C4 olefin bond of coumarin, at 1183 cm⁻¹. - ¹and 1114cm - The C–F stretching and C–O–C stretching absorption of pyranone at ¹ together indicate that the photosensitive unit of coumarin (containing trifluoromethyl and reactive double bonds capable of [2+2] photocycloaddition) is completely preserved during polymerization and thermal imidization; 1518 cm⁻¹ - The ¹ position represents absorption by the C=C stretching vibration of the benzene ring skeleton, 1241 cm⁻¹. - The ¹ position represents the absorption of the C–O–C asymmetric stretching vibration of diphenyl ether and flexible ether bridges, at 2915 cm⁻¹. - The ¹ position represents the C–H stretching vibration absorption of the flexible ether-bridged methylene group, at 3068 cm⁻¹. - The absorption at position ¹ represents the C–H stretching vibration of the aromatic ring. These results confirm that the film has achieved full imine formation and that the coumarin photosensitive unit structure is fully preserved.

[0050] 2) Verification of the relationship between exposure time and grating depth Fourteen polyimide films obtained in Example 1 were collected and covered with grating masks with a period of 10 μm. They were exposed to 365 nm ultraviolet light for 0.5, 1, 2, 3, 4, 4.5, 5, 5.5, 6, 7, 8, 10, 12, and 16 s, respectively. The grating relief depth (amplitude) of each sample was measured using an atomic force microscope. The results are shown in [Figure number missing]. Figure 4 The results show that the grating relief depth first increases and then decreases with exposure time; it increases from approximately 13 nm to approximately 132 nm in the range of 0.5–7 s, with a significant increase in the 4–7 s range; it reaches a peak of approximately 132 nm at 7 s; after 7 s, it gradually decreases and tends to approximately 86 nm (at 16 s). This decrease indicates that after 7 s, as the exposure time continues to extend, the volumetric response difference between the bright and dark areas no longer increases monotonically. Possible reasons include the cross-linking degree in the exposed area tending towards saturation, and the slight cross-linking of the dark area of ​​the grating mask after receiving a small dose of stray light, which together reduce the difference in network density and volumetric response between the bright and dark areas. Therefore, the optimal exposure window is determined to be 4–7 s, within which the grating depth is sensitive to the exposure dose, has high control precision, and is suitable for multi-level grayscale encoding.

[0051] 3) Photodimerization activity test To verify the essential difference between the diamine monomer of formula (I) of this invention and its counterpart containing 4-methylcoumarin in the [2+2] photodimerization reaction, the photodimerization reactivity of the two diamine monomers was compared and tested. The details are as follows: 1. Sample preparation Sample A: 4-(trifluoromethyl)coumarin diamine monomer with the structure of formula (II) of this invention; Sample B: Control compound 4-methylcoumarin diamine monomer. Its structural formula is completely identical to that of Sample A, except that the trifluoromethyl substituent (–CF3) at the 4-position of coumarin is replaced by a methyl group (–CH3), while the other parts (polyphenyl skeleton, flexible ether bridge, and amino groups at both ends) remain unchanged.

[0052] 2. Test Conditions Sample A and Sample B were prepared into acetonitrile (or DMF) solutions of the same concentration and irradiated under the same 365 nm ultraviolet light source (10 W power). Samples were taken at 0, 1, 2, 4, 8, 16, and 32 min to measure the ultraviolet-visible absorption spectra (UV-Vis). All conditions (concentration, solvent, light source, distance, and temperature) for the two groups of samples were kept completely consistent.

[0053] 3. Test Results Depend on Figure 6It can be seen that sample B has a characteristic absorption peak of coumarin at approximately 320 nm. This absorption peak gradually weakens with prolonged UV irradiation time, indicating that the coumarin double bonds underwent photodimerization. However, the characteristic absorption peak still maintains a high absorption intensity after 8 min of irradiation, and significant absorption can still be observed at 16 min and 32 min, indicating that its photodimerization rate is relatively slow and requires a longer time to achieve a high degree of conversion.

[0054] In contrast, by Figure 7 It can be seen that sample A has a characteristic absorption peak at approximately 310 nm. Under the same irradiation conditions, its characteristic absorption peak decreases rapidly with irradiation time, showing a significant attenuation after 4 min, further decreasing after 8 min, and stabilizing after 16 min. This indicates that its coumarin double bonds can complete the photodimerization reaction in a relatively short time, exhibiting a faster photoresponse rate and higher photodimerization efficiency. Furthermore, by comparing their photodimerization reaction rates, it can be seen that at the same exposure dose of 32 min, the characteristic absorption peak of sample A decreases by approximately 52%, while that of sample B decreases by only approximately 43%. The photodimerization efficiency of sample A is significantly higher than that of sample B, with the apparent photodimerization degree being approximately 1.2 times that of the latter at the same dose.

[0055] Secondly, during illumination, sample A exhibits a clear and strictly isosbestic point at ~275 nm, with the absorption spectra of all different exposure times converging precisely at this point (see Figure 7). The presence of the isosbestic point indicates that sample A undergoes only a single [2+2] photocycloaddition reaction under 365 nm UV illumination, cleanly converting from a monomer to a cyclobutane dimer without significant side reactions or intermediate products.

[0056] In stark contrast, although sample B's characteristic absorption at ~320 nm also decayed with irradiation time under 365 nm UV light, its UV-Vis absorption spectrum did not show a clear isoabsorption point (see Figure 6); the curves at different time points did not converge to a single point in the short-wavelength region. This phenomenon indicates that the photoreaction of the 4-methylcoumarin diamine system is not a single [2+2] photoaddition pathway, and may be accompanied by photodegradation, side reactions, or the coexistence of multiple products, making the reaction uncontrollable and the products nonspecific.

[0057] The above comparative test clearly demonstrates that there is an essential difference in the [2+2] photocycloaddition reactivity between 4-(trifluoromethyl)coumarin and 4-methylcoumarin — the former exhibits a clean, single, and efficient [2+2] photodimerization, while the latter has non-specific reaction, relatively low efficiency, and may be accompanied by side reactions. This difference stems from the fundamental influence of the strong electron-withdrawing effect of the 4-position trifluoromethyl group on the excited-state electronic structure, transition energy, and reaction path selectivity of the coumarin chromophore, rather than a simple equivalent substitution between the two.

[0058] It is precisely based on the single, clean, and efficient [2+2] photodimerization characteristic of 4-(trifluoromethyl)coumarin that the photosensitive polyimide film provided by the present invention can realize precise dose-depth regulation under mask exposure, form a multi-level grating structure free from side reaction interference, and lay a foundation for high-precision optical encryption and anti-counterfeiting applications. This essential difference also indicates that the use of 4-(trifluoromethyl)coumarin-containing diamine can achieve higher photodimerization efficiency and more stable photoresponse behavior, which is beneficial to the precise construction of a multi-level grayscale grating structure in a single-layer film. In contrast, merely replacing the 4-position trifluoromethyl group with a methyl group cannot obtain the same photoresponse characteristics and technical effects as the present invention.

[0059] Example 2: Multi-level Grayscale Optical Encoding and Decoding (1) Multi-level grayscale encoding of the "Zhong" (Chinese character meaning "middle") pattern A stepwise progressive exposure method is adopted: the polyimide film obtained in Example 1 is covered with a grating mask with a period of 10 μm and exposed to 365 nm ultraviolet light in the following manner: first, the upper part and the middle part are shielded, and only the lower part is exposed for 2 s; then only the upper part is shielded, and the middle and lower parts are exposed for 2 s; finally, the shielding is removed, and the whole is exposed for 3 s. As a result, the upper part, the middle part and the lower part accumulate exposure doses of 3 s, 5 s and 7 s respectively, forming three levels of surface relief gratings with different depths. AFM characterization shows that the grating relief depths of the three regions are about 22 nm, 78 nm and 132 nm in sequence, which increase significantly with the increase of the cumulative exposure dose (see Figure 5 (c) and Figure 5 (d)). Finally, a "Zhong" pattern encoded film is obtained. Under the experimental conditions of this example, a surface relief grating with three-level depth gradient is obtained without a development step after encoding.

[0060] (2) Hiding and revealing of information For the "Zhong" pattern encoded film obtained above, under natural light, the difference in diffractive structural color of each region is not easy to distinguish, the "Zhong" pattern is difficult to recognize, and the information is in a hidden state (as shown in Figure 5in (a)); when illuminated by directional light, since the three exposed regions have the same grating period (all equal to the mask period of 10 μm) but different grating depths, the three regions exhibit the same dominant diffraction wavelength but different diffraction efficiencies under the same observation conditions, which visually manifests as differences in brightness and saturation, creating a distinct contrast between the three levels of exposed regions, so that the Chinese character "Zhong" pattern can be clearly displayed to realize decoding (as shown in Figure 5 in (b)). Moreover, since the conditions for diffraction enhancement of gratings with different depths vary with the observation / incidence angle, the same pattern presents different structural color distributions at different angles, which further enhances the angle dependence and anti-counterfeiting non-replicability.

[0061] Example 3: Preparation of coumarin-functionalized polyimide film based on the compound of formula (Ⅲ) The structure of the compound of formula (Ⅲ) is as follows: .

[0062] The molecular formula of the compound of formula (Ⅲ) is C 34 H 32 F3N3O4. The difference from the compound of formula (II) is that the bridging atom Z of the flexible bridging chain is changed from a covalent single bond to a secondary amino group (–NH–), that is, the butyl ether chain –O–(CH2)4–O– in formula (II) is replaced by a secondary amine-containing bridging chain –O–(CH2)3–NH–(CH2)3–O–.

[0063] The monomer of formula (Ⅲ) can be synthesized according to the method for formula (II). It should be noted that in the preparation of the polyimide film: since the compound of formula (III) contains a secondary amino group that can react with dianhydride, to avoid side reactions during the polymerization process, the secondary amino group can be protected in advance with a protecting group such as Boc before polymerization, and the protecting group is removed after the synthesis of polyamic acid is completed and before imidization; alternatively, by controlling the polymerization conditions and the monomer feeding ratio, the side reaction between the secondary amine and the dianhydride can be limited to an acceptable range. All of these measures belong to conventional techniques and will not be described in detail here. Subsequently, a polyamic acid solution is prepared according to the method described in Example 1, and thermal imidization is performed to obtain a polyimide film containing the compound of formula (III).

[0064] After the polyimide film of this example is subjected to stepwise exposure through a mask under 365 nm ultraviolet light, it is expected to form surface relief gratings with different depths corresponding to the exposure dose, and exhibit a similar multi-level grayscale optical encryption effect. Specifically, the obtained polyimide film is subjected to multi-level grayscale optical encoding and decoding according to the method of Example 2, and a "Zhong" shaped encoded film with a surface relief grating having three levels of depth gradients can also be obtained. The "Zhong" pattern on this "Zhong" shaped encoded film is difficult to recognize under natural light, and can only be clearly displayed under directional light irradiation.

[0065] Example 4: Preparation of Coumarin-Functionalized Polyimide Film Based on the Compound of Formula (IV) The structure of the compound of Formula (IV) is as follows: .

[0066] The molecular formula of the compound of Formula (IV) is C 44 H 35 F3N2O4. The difference from the compound of Formula (II) is that the polybiphenyl skeleton changes from terphenyl to quinquephenyl, corresponding to m=n=2, l=1, and the amino groups at both ends change from para-position to meta-position. In the synthesis of the compound of Formula (IV), the corresponding polybiphenyl hub intermediate is 3,3''''-diamino-[1,1':4',1'':3'',1''':4''',1''''-quinquephenyl]-5''-ol (the structure is as follows):

[0067] Subsequently, a polyamic acid solution was prepared according to the method described in Example 1, and thermal imidization was performed to obtain a polyimide film containing the compound of Formula (IV).

[0068] After the polyimide film is subjected to stepwise exposure through a mask under 365 nm ultraviolet light, it is expected to form surface relief gratings with different depths corresponding to the exposure dose, and exhibit a similar multi-level grayscale optical encryption effect. Specifically, the obtained polyimide film is subjected to multi-level grayscale optical encoding and decoding according to the method of Example 2, and a "Zhong" (Chinese character for middle) patterned encoded film with a surface relief grating having three levels of depth gradients can also be obtained. The "Zhong" character pattern on this encoded film is difficult to recognize under natural light, and can only be clearly revealed under directional light irradiation.

[0069] Although the specific embodiments of the present invention have been described above, those skilled in the art should understand that these are only illustrative, and various changes and modifications can be made to these embodiments without departing from the principle and essence of the present invention. The protection scope of the present invention is limited only by the appended claims.

Claims

1. A coumarin-functionalized polyimide film for optical encryption, characterized in that, The raw materials for its preparation include dianhydride monomers, copolydiamine monomers, and polyphenylene diamine monomers containing 4-(trifluoromethyl)coumarin in a molar ratio of 10:(6~8):(2~4), wherein the polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin accounts for 20~40% of the total diamine monomers in molar percentage. The polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin has the structure shown in formula (I): Formula (I) In formula (I), Ar represents a phenyl unit, and each Ar unit is connected by a carbon-carbon single bond to form a polyphenyl aromatic skeleton; the curved bond represents the linking site, which is independently selected from the ortho, meta or para position of the corresponding Ar unit; m and n are each independently selected from 1, 2, or 3, and l is selected from 1 or 2; the two –NH2 groups are each independently located at the adjacent, meta, or para positions of the corresponding terminal Ar units; –O–(CH2) p –Z–(CH2)q–O– is a flexible bridging chain, where p and q are each independently selected from integers from 1 to 6, and Z is selected from single bonds, –O–, –S–, or –NH–. The copolydiamine monomer is selected from one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenylmethane, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane.

2. The coumarin-functionalized polyimide film for optical encryption according to claim 1, characterized in that, The dianhydride monomer is selected from one or more of the following: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(hexafluoroisopropyl)diphthalic anhydride, benzophenone tetracarboxylic dianhydride, and 4,4'-oxobisphthalic anhydride.

3. The coumarin-functionalized polyimide film for optical encryption according to claim 1, characterized in that, The polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin is at least one of the compounds shown in formulas (II), (III), and (IV): Equation (II) Formula (III) Formula (Ⅳ).

4. The method for preparing the coumarin-functionalized polyimide film for optical encryption according to any one of claims 1-3, characterized in that, Includes the following steps: (1) Under an inert atmosphere and light-protected conditions, polyphenylene diamine monomer containing 4-(trifluoromethyl)coumarin and copolydiamine monomer are dissolved in an aprotic polar solvent, and dianhydride monomer is added in batches. Polycondensation is carried out at 0~10℃ to obtain a polyamic acid solution. (2) Coating a polyamic acid solution onto a substrate and heating to remove the solvent to obtain a precursor film; (3) The precursor film was thermally imidized by gradient heating under a protective atmosphere to obtain a coumarin-functionalized polyimide film.

5. The preparation method according to claim 4, characterized in that, The aprotic polar solvent is one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide; and / or, the maximum temperature of the thermal imidization is 250~300℃.

6. A multi-level grayscale optical encoding method, characterized in that, It employs the coumarin-functionalized polyimide film for optical encryption as described in any one of claims 1 to 3, specifically including the following steps: covering the film with a grating mask, applying different exposure times to different areas of the film with ultraviolet light, causing intermolecular [2+2] photocycloid crosslinking of the coumarin side groups in the exposed area, resulting in an increase in local network density and volume shrinkage in the exposed area, and forming a surface relief grating with different depths corresponding to the exposure dose on the film surface.

7. The multi-level grayscale optical encoding method according to claim 6, characterized in that, The period of the grating mask is 5~20μm; and / or the wavelength of the ultraviolet light is 365nm, and the single exposure time is 0.5~16s.

8. The multi-level grayscale optical coding method according to claim 6, characterized in that, The surface relief gratings of different depths are formed by applying different exposure doses to different areas of the thin film. The exposure doses are achieved by one or a combination of step exposure, progressive exposure, local repeated exposure, and grayscale exposure.

9. A method for reading optically encrypted information, characterized in that, The thin film encoded by the multi-level grayscale optical encoding method according to any one of claims 6 to 8 is placed under directional light illumination. The differential diffraction structural colors generated by gratings of different depths reveal the encoded information pattern and are read by a scanning device.

10. An optical encryption or anti-counterfeiting product, characterized in that, It comprises a coumarin-functionalized polyimide film for optical encryption as described in any one of claims 1 to 3, or a film encoded using the multi-level grayscale optical encoding method as described in any one of claims 6 to 8.

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